The present invention is directed to a near-range radar distance sensor upon employment of microwave signals.
A basic task of sensor technology is the non-contacting, precise measurement of distances. Due to their rugged nature and dependability, particularly under difficult conditions of use, microwave radar sensors offer critical advantages compared to other sensors that work ultrasound or optical methods. Microwave radar sensors are therefore suitable for versatile applications such as, for example, for non-contacting measurement of range and velocity in automation technology or in automotive technology. A number of distance sensors according to the radar principle are known and described in textbooks. A particularly preferred embodiment works according to the FMCW Principle (Frequency Modulated Continuous Wave). An example of such a radar distance sensor is schematically illustrated in FIG. 2. The sensor emits a preferably linearly frequency-modulated transmission signal s (t) via the antenna A, this transmission signal s (t) being supplied by an electronically tunable microwave oscillator VCO. The reception signal e (t) received by the antenna is delayed in time relative to the transmission signal corresponding to the running time to the target and back and has a different momentary frequency dependent on running time compared thereto. A separation between the transmission signal and the reception signal in this indicated example is effected by a transmission/reception diplexer SEW. Such a transmission/reception diplexer can, for example, be formed by a circulator. An alternative is the employment of separate antennas for the transmission and reception (bistatic radar system). The measured signal mess (t) is generated by mixing the transmission signal s (t) and the reception signal e (t), for example in a mixer MI, and corresponds to the product of transmission signal and reception signal. The measured signal mess (t) arising during the mixing process has the difference frequency between the transmission signal s (t) and the reception signal e (t) as frequency. This frequency (or, respectively, the phase boost) of the measured signal mess (t) is proportional to the distance given such a sensor.
In practice, such distance sensors have technologically conditioned weaknesses in the near range, i.e. for extremely short distances to be measured. Due to unavoidable crosstalk because of the non-ideal separation of transmission signal and reception signal (e (t) contains parts of s (t)), noise/multiple reflections within the sensor, reactances of the frequency modulation onto the components of the sensor and low-frequency amplitude/phase noise of the sensor components, the sensitivity of such sensors is highly limited in the near range, i.e. the internal noise signals of the sensor superimpose a payload signal produced by a target in the near range to such an extent that the measured signal supplies no usable range information. U.S. Pat. No. 5,337,052 discloses a radar sensor that employs phase-modulated microwaves and is provided for identifying the presence of a target in a specific distance range. Existing noise is eliminated from the measurement with a special digital code of the phase modulation.
DE 195 33 125 C1 discloses an apparatus for distance measurement wherein a transit time line is employed as delay line for improving the precision of the distance measurement in the near range. A frequency-modulated signal is converted with a further microwave signal onto a high carrier frequency of the transmission signal, and the reception signal is demodulated with this frequency. The delayed signal is mixed with the frequency-modulated microwave signal to form a measured signal.
International Reference WO 83/0283 discloses a continuous wave radar apparatus with intermediate frequency formation wherein a superimposition oscillator is provided for mixing the transmission signal and the reception signal onto an intermediate frequency before the demodulation. The sensitivity of the radar apparatus is intended to be enhanced therewith.